The duration of post-traumatic amnesia (PTA), which is the period foll
owing brain injury when patients are disoriented and impaired in the a
cquisition of new information, has been used as a behavioural indicato
r of the severity of injury. This paper reviews earlier retrospective
and more recently developed prospective methods of assessing the durat
ion of PTA. Prospective assessment that commences before the resolutio
n of PTA overcomes the possible over- or underestimation of the durati
on of PTA inherent in methods that assess PTA retrospectively by clini
cal interview.